Is it time for Africa to adopt primaquine in the era of malaria control and elimination?

Primaquine is a gametocytocidal drug known to significantly reduce malaria transmission. However, primaquine induces a dose-dependent acute hemolytic anemia (AHA) in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency that has led to a limited use of the drug especially in Africa where the condition is common. The World Health Organization (WHO) now recommends a single low dose (SLD) of primaquine (0.25 mg/kg) as P. falciparum gametocytocidal without the need for prior screening of G6PD status. Adoption and implementation of SLD primaquine in Africa may probably reduce malaria transmission, a pre-requisite for malaria elimination. This review therefore, focused on the safety of primaquine for control of malaria in Africa. The literature search was performed using online database Google Scholar, PubMed, HINARI, and Science Direct. Search terms used were “malaria”, “primaquine”, “safety”, “G6PD deficiency”, “large scale” or “mass administration”. Clinical trials in many African countries have shown SLD primaquine to be safe especially in a milder African G6PD A- variant. Likewise, large-scale primaquine administrations outside Africa involving hundreds of thousands to tenths of millions of participants and with severe variants of G6PD deficiency have also shown primaquine to be safe and well-tolerated. Fourteen deaths associated with primaquine have been reported globally over the past 6 decades, but none occurred following the administration of SLD primaquine. Available evidence shows that the WHO-recommended SLD primaquine dose added to effective schizonticides is safe and well-tolerated even in individuals with G6PD deficiency, and therefore, it can be safely used in the African population with the mildest G6PD A- variant.

Page 2 of 13 Mwaiswelo et al. Tropical Medicine and Health (2022) 50:17 Background Despite recent strides in the control and elimination, malaria infection remains a major global health problem [1,2]. Malaria is one of the major causes of morbidity and mortality in Sub-Saharan Africa (SSA). In 2020 there were 241 million malaria cases and 627,000 deaths globally, and about 95% of these occurred in SSA [2]. Children under the age of 5 years and pregnant women are the most vulnerable to the infection, with children accounting for two-thirds of the malaria-related deaths [1,3,4]. Besides morbidity and mortality, malaria also negatively impacts the African economy [5][6][7][8]. Malaria control and elimination in SSA is, therefore, expected not only to alleviate morbidity and mortality but also to improve the economy.
In recent years, malaria elimination has rekindled interest in among stakeholders due to the remarkable strides gained in the past decade of enhanced control. However, the existing tools are still inadequate to eliminate the disease due to asymptomatic careers and lack of effective tools to halt transmission in most endemic areas where the burden is moderate to high. To further reduce malaria transmission and possibly reach the elimination stage, SSA countries need to adopt additional control tools including primaquine, an old but effective tool that has not been fully utilized in the continent. Primaquine offers an additional tool targeting sexual stages of the parasite lifecycle, a critical part of the comprehensive malaria elimination strategy to achieve malaria free target. Despite safety concerns especially in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency, which has been a major obstacle for use of primaquine in Africa, the drug has been used successfully in other parts of the world with severe variants of G6PD deficiency, but with no or very minimal adverse effects (AEs) [9]. The World Health Organization now recommends a single 0.25 mg/kg dose of primaquine as Plasmodium falciparum gametocytocidal without the need for prior screening of G6PD status [1,2]. Studies in SSA countries supports the safety of this single low-dose (SLD) even in individuals with G6PD deficiency [10][11][12][13][14]. SSA may therefore, adopt SLD primaquine to reduce malaria transmission and accelerate its elimination in parts of the continent that are already in the pre-elimination stage. This review, therefore, highlights studies on the safety of the WHO-recommended SLD of primaquine for control of malaria in Africa. Emphasis was also given to the safety of previous mass drug administrations (MDAs) of schizontocidal drugs plus either low or higher doses of primaquine, administered either daily or weekly over a prolonged period.

Search strategy
A literature search was performed to identify publications that reported on primaquine mass administrations or clinical settings, and its safety outcomes, particularly in G6PD deficiency. The search included primary research studies and reviews including systematic reviews and meta-analyses published from 1952 to the present. The literature search was limited to English language publications only, and it was performed using online databases Google Scholar, PubMed, HINARI, and Science Direct. The search keywords included "malaria", "primaquine", "safety", "G6PD deficiency", "large scale" or "mass administration". The identified articles were screened manually by titles, and then abstracts to assess eligibility for inclusion based on the predefined criteria. Articles which were not related to the topic of interest were excluded. Eligible articles that reported on primaquine safety, large-scale/ mass administration or clinical trial, and G6PD deficiency were identified and included in the review. Studies done in non-human subjects and articles whose full text could not be obtained were excluded.
Validity and quality of all the included studies were assessed based on the following 10 National Institute of Health criteria [15]: population, intervention, comparator, outcomes, time, setting, study design, language, publication type, and publication time frame. The criteria were stated in a form of questions that could be answered "yes" or "no".
A total of 3521 articles were identified through the databases, and of these, 382 articles related to the topic of interest were screened. Ninety-three full-text articles were assessed for eligibility, and 17 eligible articles were included in the manuscript.

Global malaria situation and control efforts
In 1900 malaria was endemic to almost all territories of the world [16,17]. Since then the world malaria map has shrunk following the successful elimination of the • With proper planning and preparation for the management of adverse effects, administration of SLD primaquine plus effective schizonticides, in a form of mass drug administration or seasonal malaria chemoprevention can be used in Africa to reduce malaria transmission.
Keywords: Malaria, Primaquine, G6PD deficiency, Safety, Control, Elimination, Africa infection in Europe, North America, and parts of Asia and North Africa [2,16]. The gains in the elimination of malaria in these territories were attributed to among other things the use of mass drug administration (MDA) using schizontocidal drugs including chloroquine, and primaquine (PQ) to eliminate P. vivax hypnozoites and kill mature P. falciparum gametocytes thus blocking transmission, and vector control using dichlorodiphenyltrichloroethane (DDT) [16][17][18][19]. No significant gains were made in Africa at the time [16,20] since the continent was not part of the global malaria eradication campaign of 1955-1969 [20]. Likewise, the emergency and spread of P. falciparum resistance against chloroquine reduced the effectiveness of the drug [18]. DDT use was also banned in 1973 due to concerns over its environmental poisoning effect [21,22]. Conversely, safety concerns against primaquine especially in G6PD deficiency limited its use especially in Africa where the prevalence of the condition is up to 33% [23][24][25]. As a consequence, currently, Africa accounts for nearly 95% of malaria cases and deaths [2]. The scale-up of malaria control started in the 2000s using tools such as vector control using long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS), chemoprophylaxis in pregnancy and under-five children, and proper management of clinical cases [26][27][28][29] witnessed a 30% reduction in global malaria incidences, and 44% reduction in malaria-related deaths in Africa between 2000 and 2015 [2,29,30]. The reduction in malaria incidences has however stalled, and the number of cases increased between 2016 and 2019 [2,30]. Furthermore, the emergency of corona virus pandemic disrupted the provision of malaria prevention, diagnosis and treatment services in 2020 that led to about 14 million more malaria cases and 47 000 more deaths in 2020 compared to 2019 [2]. Malaria control and elimination efforts have also been weakened by the inherent deficiencies of the available control tools including: low coverage of LLINs and IRS [2,31]. Furthermore, the spread of Anopheles resistance against pyrethroid [20,31,32], predominance of outdoor feeding and resting An. arabiensis as a consequence of scale-up of LLINs [33], and Anopheles behavioral changes including feeding outdoor or at early hours of the evening when most people are not under the bed net protection [33,34] are hampering the impact of LLINs and IRS. The emergency and spread of artemisinin resistance in parts of Southeast Asia [35][36][37][38], and recently in East Africa [39,40], also threatens the impact of the drug. Artemisinin derivatives are also not potent against mature P. falciparum gametocytes, thus cannot prevent the transmission of malaria [41,42]. Thus, the integration of additional control tools such as primaquine may revamp the control efforts, reduce malaria transmission and accelerate the elimination of malaria in Africa [23,43,44].

Primaquine use, hemolysis, and G6PD deficiency
Primaquine is an 8-aminoquinoline antimalarial drug first synthesized in 1946 [19] and got licensed in 1952 [45]. It has activity against intra-hepatic schizonts and hypnozoites of P. vivax and P. ovale [45]; thus it is indicated for a radical cure of P. vivax and P. ovale infection, for causal prophylaxis, and terminal prophylaxis [18,19,44,45]. Primaquine is also potent against mature P. falciparum gametocytes, a parasite stage responsible for the transmission of the infection from human to female Anopheles mosquito [19,45,46]. Whereas it is given daily for 14 days for radical cure of vivax and ovale malaria, when used as P. falciparum gametocytocidal primaquine it is given as a single dose [43]. A single dose of primaquine is sufficient to sterilize mature P. falciparum gametocytes, blocking its transmission before it kills and clears them from the circulation [43,47,48]. The drug also inhibits the production and maturation of normal sporozoites [47].
Primaquine however induces a dose-dependent hemolysis of the red blood cells (RBCs) in individuals with G6PD deficiency leading to acute hemolytic anemia (AHA) [18,43,46,47,[49][50][51][52], an adverse effect that has led to limited use of the drug especially in SSA where the condition is more prevalent [44]. The AHA can be lifethreatening since massive intravascular hemolysis with hemoglobinuria may precipitate acute renal failure especially in adults [43,45]. Besides primaquine dose, the severity of AHA also varies with the clinical status at the time of drug administration, i.e. pre-existing anemia and severity of G6PD deficiency [9,43,44]. The risk of hemolysis is markedly high in healthy than in individuals with acute malaria since the latter are often already anemic [43]. Malaria infection itself causes hemolysis with preferential destruction of older RBCs, thus in G6PD deficiency, the proportion of vulnerable older RBCs is lower in malaria than in healthy subjects [9].
The G6PD deficiency is an x-linked anomaly of the RBCs [43,53]. The anomaly is associated with hemolysis of RBCs in response to certain foods, drugs, infections, or stresses [52][53][54]. The geographical distribution of G6PD deficiency mirrors that of malaria since the condition provides some protection against the infection [9,18,45,48,53]. It is more prevalent in Africa, especially among men, with the prevalence ranging from 1 to 33% [18,19,44,45,48]. Males have only one G6PD allele, thus are either normal or hemizygous deficient, whereas females have two, thus can either be homozygous normal, homozygous deficient, or heterozygous [43,53]. Heterozygous females have a partial deficiency with some RBCs having normal levels of G6PD whereas others are deficient [43]. Hemolysis is severe in hemizygous males and homozygous females, but its severity in heterozygous varies from that in hemizygous males to that in G6PD normal individuals [43]. There are more than 180 variants of G6PD deficiency [43,44,55]. Of the variants, Mediterranean (main variant found in Europe, west and central Asia, and northern India) is the most severe deficiency and is accompanied by severe life-threatening hemolysis on exposure to oxidative agents [38,39]. The African G6PDA-variant found in SSA is the mildest [44,48], although severe hemolytic reactions can rarely occur [24,44]. In non-G6PD deficiency, primaquine is well-tolerated although it can rarely cause non-significant hemolysis [10,45].

Previous mass drug administrations of primaquine
Despite the safety concerns, primaquine has over the past 70 years contributed significantly to the global fight against malaria [19,54,[56][57][58][59], especially in lowmalaria transmission settings of Asia, the Americas, and Europe [9,48,60]. Reports on the large-scale mass drug administration (MDA) of primaquine for the control and elimination of malaria are presented in Table 1. In combination with chloroquine, a 15 mg dose of primaquine was administered for 2 weeks to prevent P. vivax relapse in some 250,000 US troops returning from the Korean War between 1950 and 1953 [54,[56][57][58]. Weekly 45 mg primaquine combined with chloroquine was also administered to the US troops in the Vietnam War [24]. In 1970's some 28 million people in Jiangsu province, China received primaquine using radical treatment regimens [60]. In Azerbaijan, Tajikistan, Northern Afghanistan, and the Democratic People's Republic of Korea (DPRK) nearly 8 million people received primaquine-MDA for prevention or elimination of P. vivax infection [9]. In Nicaragua, 1.9 million people received a 3-day regimen of primaquine plus chloroquine to control and eliminate P. vivax and P. falciparum malaria [61]. Likewise, smallscale primaquine-MDAs have been conducted in Malaysia, Cambodia, and Sumatra, Indonesia [62]. Small-scale primaquine MDAs have also been conducted in high malaria transmission-settings of SSA including in Cameroon, Mohel Island, and Tanganyika [62][63][64]. In all these MDAs, primaquine was deployed without G6PD testing and the prevalence of reported SAEs related to the drug was very low [9,[61][62][63]65]. Of note, 14 deaths associated with primaquine have been reported over the past 6 decades, and 12 of them were due to severe hemolysis [24,44,66]. One death followed a single 45 mg dose, the rest followed multiple-dose administration [43,44]. No death is known to have occurred following the administration of SLD of primaquine [43].

A single low-dose of primaquine for blocking the transmission of P. falciparum
In 2010 the World Health Organization (WHO) recommended a single dose of primaquine (0.75 mg/kg equivalent to 45 mg adult dose) for P. falciparum malaria transmission-blocking with G6PD deficiency screening [72]. However, concerns related to AHA in G6PDdeficient individuals and the limited availability of G6PD testing in the field hampered the successful implementation of the recommendation [73]. In 2012 the WHO recommended a lower dose of 0.25 mg/kg primaquine for use as gametocytocidal in falciparum malaria in low transmission settings without the need for G6PD screening [73,74]. When coadministered with effective schizontocidal drugs i.e. artemisinin-based combination therapy (ACT), a single 0.25 mg base/kg dose gives maximal transmission-blocking effect [11][12][13][75][76][77][78], thus accelerating malaria elimination strategy and reducing the rate of emergence of artemisinin-resistant malaria parasites [24]. SLD primaquine has also proven to be well-tolerated and safe in various settings with different G6PD deficiency variants [10-14, 75, 76, 79]. Comparison of hematological changes and prevalence of other adverse events associated with 0.25 mg/kg primaquine administration are presented in Table 2.

Discussion
Primaquine is the only approved antimalarial drug that can sterilize and kill mature P. falciparum gametocytes, and therefore, reduce the transmission of the parasite [19,45,46]. However, administered at higher doses of 0.5-0.75 mg/kg, the drug is associated with adverse effects particularly AHA in individuals with G6PD deficiency [18,43,46,47,[49][50][51][52]. Recently the WHO recommended a 0.25 mg/kg single-dose primaquine to be added to ACTs for the elimination of malaria in lowtransmission settings and in settings threatened by artemisinin resistance without the need for screening of G6PD status [73,74]. The low dose can also be used in higher transmission settings to reduce the transmission [23]. Despite the evidence showing SLD primaquine to be safe [10-14, 75, 76, 79], many African countries are reluctant to adopt the drug. By 2012, 20 countries worldwide included primaquine as the first-line treatment for P. falciparum in their national policy, none was in Africa [23]. Since 2012, several countries in SSA have included SLD-primaquine into policy documents [25], and mainland Tanzania adopted the drug as a treatment policy in 2020 [80], but the actual level of implementation and adherence to these policies is unclear [48]. Integration of primaquine into the malaria control toolbox and as part of the comprehensive elimination strategy may reduce

Safety
The use of primaquine has been limited especially in SSA due to a dose-dependent AHA the drug induces in individuals with G6PD deficiency, a condition occurring in the region at a prevalence of up to 33% [18,43,49].  [67], either in all individuals [60,67] or except for children aged below one year [61], or except pregnant women and infants [9], or pregnant women and children aged below 5 years [9], or in adults-only [54,68]. Importantly, in Azerbaijan and Afghanistan where Mediterranean variant prevalence varies from 0 to 38% and 5 to 10%, respectively, 15 mg primaquine was administered once daily for 14 days [9,44], and only 1% of the treated individuals experienced transient AEs including dizziness, headache, back pain, dark urine, jaundice, gastrointestinal disturbances, and mild scleral icterus [9]. Similarly, in Tajikistan where Dushanbe is the predominant variant, primaquine was administered with only 1% of treated individuals developing AEs [9]. On the other hand, the African G6PD A-variant is the mildest of all the G6PD deficiency variants [44,48]; thus it is relatively resistant to primaquine-induced hemolysis [24,[43][44][45]66]. In G6PD A-, variant older RBCs succumb first to primaquine-induced hemolysis since they have the lowest content of G6PD, but young reticulocytes replacing hemolyzed RBCs have greater G6PD content, and are considerably more resistant to hemolysis [9,45,65,83]. This phenomenon leads to a mild and self-limiting hemolysis [45,46,49]. Therefore, the WHO-recommended SLD primaquine is expected to cause only mild and self-limiting hemolysis in the African G6PD A-variant. Besides AHA, primaquine is also associated with abdominal pain when taken on an empty stomach [9,19,24,45,49,55]. The drug also triggers nausea, vomiting, and mild diarrhea [18,48]. The severity of the gastrointestinal AEs is related to the dose administered [49]. A 15-mg dose of primaquine given on an empty stomach is associated with only mild abdominal pain, whereas higher doses (30 mg or 45 mg) are associated with mild to severe abdominal discomfort with nausea and vomiting [49]. Taking food before primaquine administration can alleviate the AEs [9,24,45,46]. Food intake also increases the oral availability of primaquine hence improving the drug's efficacy [19]. On the other hand, primaquine also induces methemoglobinemia, an abnormal accumulation of methemoglobin [19,49]. Methemoglobinemia is common in individuals with nicotinamide adenine dinucleotide hydrogen methemoglobin reductase enzyme deficiency [9,19,24]. This enzyme deficiency is however rare [19]. Methemoglobinemia usually occurs with therapeutic or prophylactic primaquine regimens [19,45], but very rarely is dangerous [44]; however severe cases may be treated with 300 mg of methylene blue [49]. Neuropsychiatric side-effects such as depression and psychosis [19], hypersensitivity reactions [45], and visual disturbances [49], have also been reported following the intake of primaquine although they are rare. Primaquine is also contraindicated in pregnancy as it increases the risk of intravascular hemolysis to the mother and fetus [18]; however, it is safe for use in breastfeeding women [45,84].

Primaquine implementation strategies
Despite the significant role played by primaquine MDAs in the reduction and elimination of malaria in low-transmission settings outside Africa, the same impact may probably not be achieved in SSA in a short time. One major reason is that majority of the SSA countries are still in the malaria control phase [25], thus primaquine implementation strategies that worked in low-transmission settings may probably not work in SSA. Nonetheless, in addition to other malaria control tools the SLD primaquine may probably play a significant role in reducing malaria transmission in SSA, an important prerequisite for elimination of malaria in the region. The SLD primaquine may be implemented using three major strategies namely, i) SLD primaquine plus an effective schizonticides MDA, ii) SLD primaquine added to seasonal malaria chemoprevention (SMC), and iii) addition of SLD to an effective ACT for routine treatment of clinical cases attending the health facilities. The SLD primaquine plus effective schizonticides MDA involving the whole population has potential to substantially reduce malaria transmission and accelerate elimination in SSA. This is because the majority of the individuals in endemic settings of SSA are semi-immune to malaria and carry asymptomatic infection. Due to this, they do not seek the medical attention, whereas relatively few individuals with weak immunity particularly the under-five children and pregnant women are the ones who are likely to develop symptoms and seek medical attention [85][86][87][88]. These asymptomatic individuals act as reservoirs of the infection in the population [87,88]. Therefore, MDA involving the whole population may probably easily capture the asymptomatic individuals and reduce malaria transmission significantly. Previous primaquine MDAs conducted in high transmission settings of Cameroon, Comoros Island, and Tanganyika [62][63][64] and were able to reduce although failed to interrupt the transmission. However, the reduction of malaria transmission is a prerequisite for elimination of the infection, thus MDA should be implemented despite the failure of the strategy to interrupt the transmission. A significant reduction of malaria transmission in SSA will be an important step in realizing malaria elimination target by 2030. Nonetheless, prolonged primaquine-MDAs in high transmission settings may increase the likelihood of development of parasite resistance against the drug. The SMC is another strategy that may be implemented especially in settings where malaria transmission is highly seasonal [23]. The SMC using sulfadoxine-pyrimethamine plus amodiaquine has been widely implemented in the Sahel region and it has substantially reduced the incidences of malaria infection in under-five children [89][90][91][92]. In this region, SLD primaquine can be added to sulfadoxine-pyrimethamine plus amodiaquine to increase the impact of the SMC; however, this will require further investigation to ascertain the safety of the strategy. A similar strategy has been used in China whereby seasonal primaquine plus chloroquine MDA was administered to almost 30 million people and led to a significant reduction of malaria incidences [60]. SMC using ACTs plus primaquine may also be implemented in parts of East and Southern Africa that have highly seasonal malaria transmission. For instance, southern regions of mainland Tanzania have highly seasonal malaria transmission [93], therefore, may use the primaquine plus ACT SMC strategy to further reduce the transmission. Furthermore, some countries including Algeria, Botswana, Cape Verde, Eswatini, Namibia, South Africa, and Zanzibar are nearing malaria elimination stage [25], thus they may use primaquine as a component of SMC to accelerate the elimination of the infection. The SLD primaquine may also be added to ACTs to treat the uncomplicated P. falciparum cases detected at the health facilities. This strategy is however useful in settings where malaria prevalence is close to elimination. However, in high transmission settings administration of SLD primaquine to every clinical malaria case cannot have any significant impact since the majority of malariainfected individuals are asymptomatic [86,88]. On the other hand, the WHO recommendation is that SLD primaquine should be administered without the need to screen for G6PD deficiency [73,74]. However, the adoption of SLD policy varies from one country to another, with some African countries such as Mauritania, Mayotte, Sao Tome and Principe, and Cape Verde requiring testing for G6PD deficiency before administering primaquine whereas other countries such as Botswana, Namibia, and Madagascar do not require the testing [25]. The mandatory requirement to test for G6PD deficiency is hampering the efforts to adopt SLD primaquine. Nonetheless, the available evidence on SLD primaquine safety in G6PD deficiency is adequate, therefore, in countries where G6PD deficiency testing is mandatory efforts should be made to remove the restrictions to improve the scale-out of primaquine.

Limitations
The review had limitations including that only English language articles were included since no one among the authors is fluent in others languages such as French and Spanish, only free databases were used for literature search since the review was not funded, and not all the studies included met all the inclusion criteria.

Conclusion
Available evidence shows that the WHO-recommended SLD primaquine dose added to effective schizonticides is safe and well-tolerated even in individuals with G6PD deficiency, and therefore, it may safely be used in the African population with the mildest G6PD A-variant. Adoption and roll-out of SLD primaquine in Africa can substantially reduce malaria transmission, an essential prerequisite for the elimination of the infection in the continent.